mon: MDSMonitor: don't return -EINVAL if function is bool

mon: MDSMonitor: don't return -EINVAL if function is bool

Returning -EINVAL on a function that expects bool and the error code to
be in a variable 'r' can only achieve one thing: if this path is ever
touched, instead of returning an error as it was supposed to, we're
returning 'true' with 'r = 0' and, for no apparent reason, the user will
think everything went smoothly but with no new fs created.

Signed-off-by: Joao Eduardo Luis <joao@redhat.com>
(cherry picked from commit 91b2a2b69b198567d42f45f75b7f7ea559f89150)